LONDON (Reuters) - Since he ditched Marlboro Lights five years ago, Daniel's fix is fruit-flavored nicotine gum that comes in neat, pop-out strips. He gets through 12 to 15 pieces a day and says he has "packets of the stuff" stashed all over. But he doesn't see himself as a nicotine addict.

Like many people, Daniel believes nicotine gum is far less harmful for him than smoking. Doctors worldwide agree. By giving up cigarettes, they say, Daniel has removed at least 90% of the health risks of his habit.

Even so, the possibility that people can be addicted to nicotine, but not die from it, is at the heart of a growing debate in the scientific community. Scientists don't doubt nicotine is addictive, but some wonder if a daily dose could be as benign as the caffeine many of us get from a morning coffee.

It's a debate that has been aggravated by the rising popularity of electronic cigarettes - tobacco-free gadgets people use to inhale nicotine-laced vapor, which have helped some people quit smoking. The idea of nicotine as relatively benign goes against the negative image of the drug that built up over the decades when smoking rose to become an undisputed health threat.

Psychologists and tobacco-addiction specialists, including some in world-leading laboratories in Britain, think it's now time to distinguish clearly between nicotine and smoking. The evidence shows smoking is the killer, not nicotine, they say.

"We need to de-demonize nicotine," said Ann McNeill, a professor of tobacco addiction and the Institute of Psychiatry, Psychology and Neuroscience at King's College London, who has spent her career researching ways to help people quit smoking.

She wants people to understand the risks are nuanced - that potential harms lie on a curve with smoking at one end, and nicotine at the other. People who don't see that may hesitate to seek help stopping smoking, or try to restrain their intake of nicotine replacement therapy (NRT). That can make it harder to quit.

Some studies show nicotine, like caffeine, can even have positive effects. It's a stimulant, which raises the heart rate and increases the speed of sensory information processing, easing tension and sharpening the mind.

All this raises other questions: Could nicotine prime the brains of young people to seek harder stuff? Or, in an ageing society, could its stimulant properties benefit people whose brains are slowing, warding off cognitive decline into Alzheimer's and delaying the progression of Parkinson's disease?

So far the answers aren't clear. And the divide is as political and emotional as it is scientific.

RELATIVE HARMS

McNeill says her work is, in part, to honor the legacy of her former mentor at King's, British psychiatrist Mike Russell. About 40 years ago, Russell was one of the first scientists to suggest that people "smoke for the nicotine, but die from the tar" - an idea that helped lay the ground for the NRT business of gums, patches, vaporizers and now e-cigarettes.

Some scientists note Russell's insight has been misused by the tobacco industry. For decades, companies' false promises of "light" cigarettes helped lure more smokers, says Mike Daube, professor of health policy at Curtin University in Australia. "We have seen more than six decades of tobacco industry distraction products, promotions and deceptions," he says. "They reveled in advertising that implied both reduced risks and even health benefits."

Smoking kills half of all those who do it - plus 600,000 people a year who don't, via second-hand smoke - making it the world's biggest preventable killer, with a predicted death toll of a billion by the end of the century, according to the World Health Organization.

Few doubt that nicotine is addictive. How quickly it hooks people is closely linked to the speed at which it is delivered to the brain, says McNeill. The patch is very slow; gum is slightly quicker. But there is no evidence as yet that significant numbers of people are addicted to either. Daniel, who works long hours in London's financial district, says he chews less on weekends when he's relaxing, doing sport and hanging out with his kids.

One reason smoking is so addictive is that it's a highly efficient nicotine delivery system, McNeill says. "Smoking a tobacco cigarette is one of the best ways of getting nicotine to the brain - it's faster even than intravenous injection." Also, tobacco companies used various chemicals to make the nicotine in cigarettes even more potent.

Pure nicotine can be lethal in sufficient quantities. There is some evidence it may lead to changes in adolescent brain development, especially to the part responsible for intelligence, language and memory.

Stanton Glantz, a professor of tobacco at the University of California, San Francisco, says the younger kids are when they start using nicotine, the more heavily addicted they get. "This is likely because their brains are still developing," he said.

Countering that, others say studies have focused on animals and that in any case, nicotine should not be available to under-18s. Michael Siegel, a tobacco control expert and professor at Boston University, says that in the few studies so far, such effects have been seen only in smokers, not smoke-free nicotine users.

Elsewhere, studies have looked at nicotine's potential to prevent Alzheimer's disease, and to delay the onset of Parkinson's.

A study in the journal Brain and Cognition in 2000 found that "nicotinic stimulation may have promise for improving both cognitive and motor aspects of Parkinson's disease." Another, in Behavioral Brain Research, suggested "there is considerable potential for therapeutic applications in the near future." Other work has looked at the stimulant's potential for easing symptoms of attention deficit hyperactivity disorder (ADHD).

In Sweden, many people get their nicotine from sucking smoke-free tobacco called "snus." Research there has put rates of lung cancer, heart disease and other smoking-related illness among the lowest in Europe.

"FUNCTIONAL ADDICTION"

Even so, the idea of "safe nicotine" has not caught on.

Marcus Munafo, a biological psychologist at Britain's Bristol University, says public health campaigns of the 1970s and 1980s bound nicotine, addiction and cigarettes tightly together to hammer home smoking's harms. Those associations may blur the potential for cleaner nicotine to lure smokers away from cigarettes.

Munafo is questioning the notion that a nicotine addiction is, in itself, bad. At a "smoking laboratory" in Munafo's department, people who are still hooked on cigarettes smoke under controlled conditions. At the moment, researchers are studying genetic differences in how deeply people inhale, as part of a project analyzing people's needs and responses to nicotine.

"Should we really be that bothered about addiction in and of itself, if it doesn't come with any other substantial harms?" said Munafo. "It's at least a discussion we need to have."

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